1. Field of the Invention
The present invention relates to an optical pickup actuator, and more particularly, to an optical pickup actuator having an improved structure which is driven along a drive axis different from the optical axis of an objective lens and removing factors affecting the generation of a subsidiary resonance due to the leakage magnetic flux to improve sensitivity.
2. Description of the Related Art
In general, optical pickup assemblies are adopted in optical recording and/or reproduction apparatuses to perform recording and/or reproduction of information in a non-contact manner with respect to a disk, which is a recording medium loaded on a turntable, while moving across the disk.
The optical pickup assemblies include actuators to drive an objective lens in a track direction and a focus direction so that a light spot is focused at a desired track position of an optical disk. However, since portable personal computers such as laptops need to be manufactured thin and light, the entire volume of the system is restricted, and thus an actuator adopted by the system needs to be slim. A reflective mirror to make a light beam proceed toward the objective lens is adopted in the optical pickup. To meet the need of a slim actuator, an asymmetrical actuator has been suggested, in which a drive axis of an actuator is different from an optical axis of an objective lens so that the distance between the objective lens of the optical pickup and the reflective mirror can be reduced. An example of the asymmetrical actuator is disclosed in U.S. Pat. No. 5,684,645.
Referring to FIGS. 1 and 2, a conventional optical pickup actuator 10 has a holder 14 at one side thereof and a focusing coil 18 wound around along the outer circumferential surface of a bobbin 12 having a first guide hole 16a formed at the center thereof. A pair of tracking coils 15 are wound at one side of the bobbin 12. Also, a second guide hole 16b is formed in a moving portion 17 where an objective lens 11 is installed at one side thereof. The bobbin 12 is accommodated in the second guide hole 16b. Here, a U-shaped yoke 31 is inserted into the first and second guide holes 16a and 16b. A magnet 32 is provided at one side of the yoke 31 to face the tracking coils 15.
The moving portion 17 is supported by two pairs of suspensions 13a and 13b, with each suspension 13a and 13b having one end thereof fixed to the holder 14 and the other end fixed to the moving portion 17. The pairs of suspensions 13a and 13b are on opposite sides of the moving portion 17. The moving portion 17 and the bobbin 12 are coupled to be moved together.
When current is applied to the focusing coil 18 and the tracking coils 15, the focusing coil 18 and the tracking coils 15 receive forces by the electromagnetic interoperation between the magnet 32, the focusing coil 18, and the tracking coils 15, so that the moving portion 17 is moved. The direction in which the focusing coil 18 and the tracking coils 15 receive the forces is determined by Fleming's left hand rule.
Thus, when the electromagnetic force acts by the interoperation between the magnet 32, the focusing coil 18, and the tracking coils 15, the bobbin 12 is moved in the focusing direction F or the tracking direction T. Accordingly, as the moving portion 17, coupled to the bobbin 12, moves together with the bobbin 12, the objective lens 11 moves so that the position at which a light spot is focused is adjusted.
FIGS. 3A and 3B are schematic views illustrating the electromagnetic interoperation between the focusing coil 18 and the magnet 32. Here, the focusing coil 18 can be divided into an inner focusing coil 18a disposed inside the yoke 31 and an outer focusing coil 18b disposed outside the yoke 31. However, while the inner focusing coil 18a receives an electromagnetic force by the interoperation with the magnet 32, the outer focusing coil 18b, blocked by the yoke 31, is not affected by the magnet 32. Actually, as indicated by the dotted lines of FIG. 3A, the magnetic lines of force produced by the magnet 32 are spread widely at the edges of the magnet 32 so that the magnetic flux is leaked outside the yoke 31.
The leakage magnetic flux affects the outer focusing coil 18b. In FIG. 3A, the arrows from the focusing coil 18 show the size and direction of forces applied to the focusing coil 18 according to the distribution of the magnetic lines of force by Fleming's left hand rule. The outer focusing coil 18b receives a force generated by the leakage magnetic flux, which causes an unbalanced distribution of forces from the viewpoint of the whole focusing coil 18. That is, as shown in FIG. 3B, since a force Fu applied to the inner focusing coil 18a and a force Fd applied to the outer focusing coil 18b are not balanced, a pitching mode in which the bobbin 12 and the moving portion 17 are swayed back and forth, as indicated by an arrow P in FIG. 3B, is generated.
Also, the outer focusing coil 18b is not used for the focusing operation but only increases weight and resistance of a winding coil, causing deterioration of sensitivity of the actuator. Thus, the outer focusing coil 18b becomes an obstacle with respect to a high speed following capability according to a high multiple speed of a disk.
Meanwhile, when the bobbin 12 moves in the track direction T by the tracking coils 15 (FIGS. 1 and 2), since the center point of the movement is not congruous with the center (G) of gravity, a rolling mode is generated. As illustrated in FIG. 4A, when the bobbin 12 (see FIG. 1) stands still, the center (G) of gravity of the entire actuator 10 and the center (H) of movement are congruous. In the figure, the arrows denote the size and direction of a force applied to the tracking coils 15 by the magnet 32. The magnitude of the force received by the tracking coils 15 depends on the current flowing in the tracking coil 15 and the amount of magnetic flux. Assuming that the current is constant, the size of the force received by the tracking coils 15 depends on the amount of magnetic flux only. However, the magnetic flux is the largest at the central portion of the magnet 32 and decreases from the central portion of the magnet 32 to the edge thereof.
When the tracking coils 15 are at a neutral position, as shown in FIG. 4A, since the magnetic flux is distributed symmetrically with respect to the tracking coils 15, the center (G) of gravity and the center (H) of movement are congruous.
However, as illustrated in FIG. 4B, when the bobbin 12 (see FIG. 2) moves upward due to the focusing coil 18, the force of the magnet 32 affecting the tracking coils 15 is biased to the lower side of the tracking coil 15. Thus, since the tracking force on the lower side of the bobbin 12 is greater than the tracking force on the upper side thereof, a rotational moment is generated in a direction R1.
To the contrary, as illustrated in FIG. 4C, when the bobbin 12 moves downward by the focusing coil 18, the force of the magnet 32 affecting the tracking coils 15 is biased to the upper side of the tracking coil 15. Thus, since the tracking force on the upper side of the bobbin 12 is greater than the tracking force on the lower side thereof, a rotational moment is generated in a direction R2.
As a result, as illustrated in FIG. 4D, since the center (H) of movement and the center (G) of gravity of the tracking coil 15 are not congruous according to the focusing operation of the bobbin 12, a rolling mode in which the bobbin 12 rolls in directions R1 and R2 is generated.
A rotation/vibration mode, such as the pitching mode and a rolling mode has an ill effect on the phase and displacement of a basic frequency during the focusing and tracking operations. Accordingly, an optical signal is deteriorated. Thus, when the physical size of the magnet 32 is increased to increase the density of magnetic flux so that AC sensitivity is improved, since a subsidiary resonance is produced, there is a limit in increasing the density of magnetic flux. Further, during high multiple speed of high density optical recording and/or reproduction apparatuses, since the pitching mode and rolling mode become more serious, an optical pickup actuator suitable with high multiple speed optical recording and/or reproduction apparatuses is required.